In modern disk drive assemblies, the mechanical clearance between the surface of the magnetic disk which stores data and the read/write head which transfers data to and from the disk is extremely small, i.e., about 10 microinches. Any particulate contamination on the disk surface or in the region of the read/write head is likely to cause a malfunction of the assembly. Consequently, the computer disk and the head must be enclosed and must be protected against contamination from the outside environment. The motor which drives the disk shaft and the bearings which support the disk shaft are sources of contamination due to their mechanically movable parts. However, it is generally inconvenient to enclose the motor. Thus, a magnetic fluid exclusion seal is typically employed on the drive shaft to prevent contaminants from the motor and the bearings and from the external environment from reaching the interior of the disk assembly along the drive shaft.
The trend toward miniaturization of disk drives has required thinner and thinner drives, thereby severely limiting the axial space available for magnetic fluid seals and bearings, and requiring designs with a very small axial length or thickness. Magnetic fluid seals for use in small disk assemblies such as 51/4 inch and 31/2 inch spindles are typically required to have an axial thickness on the order of 1 mm, while disk shaft bearings are typically required to have an axial thickness on the order of 5 mm.
A variety of bearing assemblies and separate magnetic fluid seals to perform the exclusion function have been disclosed in the prior art. Although such configurations function satisfactorily, they do not meet the seal thickness requirements of present disk drives. For example, U.S. Pat. No. 4,293,137, issued Oct. 6, 1981 to Ezekiel, discloses a seal configuration wherein annular magnets are positioned at opposite ends of a bearing with a magnetic fluid seal in the gap between each annular magnet and the shaft. This configuration is not suitable for use in applications requiring a very small axial dimension such as the disk drive assembly described above, since a reduction in magnet thickness would reduce the magnetic field below the level required for holding the magnetic fluid in the seal area. Bearing and seal assemblies wherein a seal is axially positioned between two bearings which support a shaft are disclosed in U.S. Pat. No. 4,526,380, issued July 2, 1985 to Raj et al and U.S. Pat. No. 4,407,518, issued Oct. 4, 1983 to Moskowitz et al. Although the disclosed seals do not add to the axial length of the assembly, the seals do not exclude contaminants generated by the bearing adjacent to the disk area.
It a general object of the present invention to provide improved magnetic fluid seal assemblies.
It is another object of the present invention to provide magnetic fluid seal assemblies with a very small axial dimension.
It is a further object of the present invention to provide magnetic fluid seal assemblies which are easily manufactured and which are low in cost.